Just to reiterate...
I would really like to see us communicate this decision, with an
appropriately persuasive argument, both to the glibc authors and to the
standards committees, and treat this as a *transitional* hack, which is
needed until glibc (and any other libc if there are any?) can remove the
erroneous nonnull declarations from their headers, and everyone has
upgraded.
I’m happy to handle at least the standards side of this; I don’t really have great connections on the libc front, but happy to help folks do that.
Are there blockers to landing https://reviews.llvm.org/D30806 ?
I’m committed to following through with the respective standards bodies and anyone else we feel is prudent, but I would somewhat like to unblock the optimization work in this area in parallel as I suspect that the communication and establishment of a rational plan will require a lot of time.
I'm in support of unblocking the LLVM side optimization changes. I'm definitely not qualified to speak towards the direction of the clang changes though. 
Philip
FYI, this is landed in r298491; thanks for the reviews.
I’m hoping to have papers for standards bodies (and also useful for libc maintainers) by next week.
Sounds great!